D.C. chopper control device for electric motors

ABSTRACT

The invention relates to a control device for D.C. motors operating by chopping the supply current. Said device comprises a voltage-frequency converter, a frequency divider, a monostable multivibrator, an analog-digital converter, a pulse down-counter and a bistable flip-flop, arranged to chop the direct current supply by means of a hit or miss control signal of which the cyclic ratio is a linear function of a control voltage U.

The present invention relates to the supply of D.C. to electric motorsand has specific reference to a device for controlling a D.C. motor bychopping its energizing direct-current, with concomitant variation ofthe chopping frequency and conduction time.

One of the known methods of controlling a D.C. motor consists incontrolling the current flowing through this motor. In these methods, itis customary to use means for chopping the voltage across the motorterminals in order to produce a chopped or hatched current of which thechopping period denoted T comprises two phases, i.e. a phase having atime τ during which the voltage fed to the motor is equal to the maximumvoltage available, therefore, a phase during which the current has alsoits maximum value I_(M), and another phase having a time T - τ duringwhich the voltage across the motor terminals is zero, the current alsohaving zero value. The average current value I during one period is thusI = I_(M) × (τ/t) , and the controlling of this average current consistssimply in controlling the ratio τ/T, also referred to as the cyclicratio of the voltage controlling the chopping members. Therefore, amotor control device is obtained by generating a power element controlsignal of the "hit or miss" (or "on - off") type of which the cyclicratio is a linear function of a reference magnitude, generally a controlvoltage U.

To obtain this function two types of solutions are utilized in existingdevices. A first method of controlling the cyclic ratio of a "on - off"control signal consists in generating such signal of fixed frequency F,of which the conduction time τ varies in proportion to the controlvoltage U. Thus, the motor is fed with a pseudo alternating current offrequency F and, to ensure a proper operation irrespective of the motorvelocity, this value F must be relatively high. As a result,considerable losses are experienced, since each switching of the powermeans is a cause of energy dissipation. Another known method ofcontrolling the cyclic ratio consists in generating a signal of fixedconduction time and having a frequency F proportional to the controlmagnitude U. In this case, since the current is expected to vary betweena maximum value and the zero value, the frequency must also be close tozero, which causes a noisy operation of the motor attended by aninsufficient progressiveness of the control device at low currentvalues.

Now, it is the essential object of this invention to provide a thirdtype of device for controlling a D.C. motor by the simultaneousvariation of frequency and conduction time, both variations being sorelated to each other that the linearity between the control voltage Uand the cyclic ratio of the current, and therefore its average value,are preserved. Due to the possibility thus afforded of varying thecurrent frequency through a limited range, it is possible with thisdevice to improve the motor efficiency at low current values whilepreserving a frequency value sufficient for improving the flexibility ofoperation of the controlled motor. The principle on which the deviceaccording to this invention is based may be summarized as follows:

The value F of the chopping current is set by means of a deviceresponsive to the control voltage, the response of this device being inturn of any desired character. The time corresponding to the period ofthis current is divided into N equal portions. An analog-digitalconverter generates from the control voltage U a number n proportionalthereto, the total variation of this number n being spread over therange O to N. A device makes the power members conductive during n amongthe N portions of the control signal period. As a result, the currentfrequency assume the value F (which frequency may vary in any desiredmanner as a function of U) with a cyclic ratio n/N, i.e. proportional toU since N is a constant.

The advantages resulting from the device of this invention pertain toseveral types:

the frequency variations may be determined independently of the actualvariations of the current proper, thus affording a lower dynamics of thefrequency variations than that of current variations. Under theseconditions, the efficiency is improved since with a low current thefrequency is reduced and the problems arising in case of abnormally lowcurrent are safely eliminated for frequency decreases at a slower ratethan current while preserving a finite value with a zero current value;

from a similar point of view, one may contemplate a frequency valuecontrolled as a function of the real motor velocity, instead of as afunction of the reference value of the current, thus affording anall-round high-efficiency motor operation;

in the device of this invention the variation of the current cyclicratio as a function of the control voltage U is linear, thus permittingof inserting the device in a closed loop (with real current measurementand feedback), without any apprehension for instabilities possiblycaused by non-linear devices;

the discontinuous character of the device, as a consequence of thedigital treatment of date, permits of adapting the adjustment precisionto the desired degree of accuracy for the system incorporating thecontrolled motor.

In fact, the higher the number N, the lower the variation increment inthe cyclic ratio and therefore the greater the adjustment fineness. Nowthe complexity and therefore the cost of the system are closely relatedto the magnitude of said number N. Thus, the best possible cost toperformances ratio can be achieved.

Finally, due to this same digital treatment it is possible tocontemplate a integrated-mass form of embodiment for increasingreliability and reduce costs.

The applications of this device are those requiring the adjustment ofthe point of operation of an electric motor fed from a source of directcurrent. Thus, more particularly, any devices for controlling the motorsof electric vehicles may be equipped with the system of this invention.

Other features and advantages of this invention will appear as thefollowing description proceeds with reference to the accompanyingdrawings, in which:

FIG. 1 illustrates in block diagram the arrangement for controlling aD.C. motor and incorporating the device of this invention;

FIG. 2 is another block diagram showing more particularly the device ofthis invention;

FIG. 3 illustrates a typical form of embodiment of the voltage-frequencyconverter of FIG. 2;

FIG. 4 is a diagram illustrating a typical form of embodiment of theanalog-digital converter of FIG. 2;

FIG. 5 illustrates the diagram of the signals produced by the device ofFIG. 4; and

FIG. 6 is a similar diagram of the signals produced by the device ofthis invention.

Referring first to FIG. 1, the motor 1 is fed through the medium of achopper 2 from a storage battery 3. Inserted in the power circuit is ashunt 4 for measuring the current I flowing through the motor 1. Acontrol member 5 is provided for displaying a reference value of saidcurrent I and generating a voltage Uc proportional thereto. This voltageUc as well as a voltage Ui are fed to a comparator 6. The voltage Ui istaken from the output of a low-pass filter 7 supplied likewise throughthe shunt 4. Therefore, this voltage Ui is the display of the realcurrent I. From the output of said comparator 6 the voltage U isdirected through a device 8 constituting the subject-matter of thisinvention. This device 8 is adapted to generate a signal S forcontrolling the chopper 2, i.e. a "hit or miss" control signal havingpredetermined frequency and cyclic ratio.

As shown in FIG. 2 illustrating diagrammatically by way of example thedevice 8 of this invention the control voltage U is fed to avoltage-frequency converter 9 delivering a signal of frequency F₁. Thissame voltage U is also fed to an analog-digital converter 10 deliveringa binary number "a" proportional to U in a parallel form, via its outputbits. In the form of embodiment illustrated and described herein thenumber "a" comprises eight bits, this accounting for the fact that theconverter has eight outputs designated by the reference symbols 11-0 to11-7. This number "a" is also applied to the predetermining inputs 12-0to 12-7 of a down-counter 13. This down-counter 13 may advantageously beconstructed by using commercially available integrated elements, such asthe SN 74 193 circuits manufactured by Texas Instruments. In the form ofembodiment described herein, two counters of this type are used, andreference may be made to the manufacturer's instruction booklets forconnecting them in actual practice.

The down-counter 13 to which said number "a" is fed comprises a clockinput 14 to which the signal of frequency F₁ delivered by converter 9 isfed, another loading input 15 and a so-called retaining output 16. Whena pulse is fed to input 15 the number "a" is recorded in the flip-flopsof the down-counter. When from that time "a" pulses of the signal fed to14 have been actually down-counted, a pulse is displayed at theretaining output 16.

The signal F₁ from converter 9 is fed to a frequency divider 17delivering at its output a signal having a frequency F₁ /A, in which Ais the maximum value assumed by "a" when U varies. This signal is fed tothe loading input 15 of down-counter 13 via a monostable multivibrator18. This signal available at the output of said monostable multivibratoris also fed to the input of a bistable flip-flop 19 of which the output20 will thus become operative. The same flip-flop has a resetting input21 to which the signal from down-counter 13 is fed.

In the form of embodiment illustrated in FIG. 3, the voltage U is fed tothe input of an integrator comprising a resistor 22, a capacitor 23 andan operational amplifier 24. The voltage from the output 25 of thisamplifier is therefore a slope as a function of the slope time which isproportional to U. This voltage is fed to the reversing input 26 of acomparator 27 adapted to be tipped when this voltage rises to apotential at least equal to the reference voltage U_(ref) fed to itsnon-reversing input 28. The tipping of comparator 27 causes a pulse tobe generated at the output of a monostable multivibrator 29 to which itis connected. This pulse, amplified by an amplifier 30, is fed via adiode 31 and a capacitor 32 connected in parallel to the diode to afield effect transistor 33 which short-circuits the capacitor 23 topermit the resumption of the cycle. The frequency F of the pulses at theoutput Sc of monostable multivibrator 29 is the reverse of the chargingtime of capacitor 23, i.e.: ##EQU1## In this relationship, U is thevalue of the input voltage, U_(ref) the value of the voltage fed to theinput 28 of comparator 27, C the value of capacitor 23, and R the valueof resistor 22. Therefore, the frequency of the output signal Sc fromthe output of monostable multivibrator 29 is proportional to the inputvoltage.

As shown in FIG. 4, the voltage U of which the value is to be convertedinto a binary number is fed to the input of a voltage-frequencyconverter 34 similar to the one illustrated in FIG. 3. This convertergenerates at its output 35 a signal having a frequency F₂ such that F₂ =kU, wherein k is a constant coefficient. This signal is fed via an ANDgate 38 to the counting input 36 of a binary counter 37 consisting of abistable flip-flop or, still better, of integrated elements of the SN7493 type manufactured by Texas Instruments. Similar circuits areavailable from many other manufacturers. The instruction booklets issuedby these manufacturers permit of assembling these elements in series inorder to obtain a counter having 256 positions (i.e. eight binaryoutputs designated by the reference numerals 39-0 to 39-7) as requiredfor the purposes of this invention. These eight outputs are eachconnected to the input of a storage bistable flip-flop 40. Alsoavailable commercially are such bistable flip-flops mounted in theregister form comprising several bistable flip-flops. Thus, forinstance, the circuit SN 74 100 manufactured by Texas Instrumentsprovides eight bistable flip-flops in a common case. Similar elementsmay also be obtained from other manufacturers. The register 40 utilizedin the form of embodiment described herein will thus comprise eight datainputs and with the eight outputs 41-0 to 41-7 the binary number fed tothe inputs are permanently available when a pulse is fed to the registerinput 42. A timer is obtained as follows: a capacitor 43 and a resistor44 are connected in series between the ground and the positive terminalof a supply source. The point common to said resistor 44 and capacitor43 is connected to the emitter of a unijunction transistor 45 having onebase connected to the positive terminal of the source of current via aresistor 46, the second base of said transistor being grounded viaanother resistor 47. The voltage is taken from one terminal of resistor47 and fed via another resistor 48 to the base of a transistor 49 havingits emitter grounded and its collector connected to the positive supplyterminal via a further resistor 50. Thus, a pulse generator is obtainedof which the recurrent frequency is bounded to the values of capacitor43 and resistor 44. These pulses available at a point 51 common toresistor 50 and transistor 49 are applied on the one hand to amonostable multivibrator 52 delivering at 53 a pulse of duration D fromthe leading edge of the control pulse and on the other hand to theresetting input 54 of binary counter 57 via a reversing switch 55. Thepulse available at the output 53 of monostable multivibrator 52 iscoupled to the second input of said AND gate 38 and also to a secondmonostable multivibrator 56 which, at the trailing edge of the controlpulse, delivers a brief pulse transmitted to the write-in or recordinginput 42 of storage register 40.

The mode of operation of the analog-to-digital converter of FIG. 4 willnow be explained with reference to the signal diagram of FIG. 5.

Upon each pulse generated at 51 (FIG. 5, line 5A) the binary counter 37is reset. The output pulse of monostable multivibrator 52 starts at theleading edge of the pulse at 51 and has a duration D (FIG. 5, line 5B).During this time period, the AND gate 48 permits the passage of thesignal F₂ emitted by the converter 34, and the counter 37 begins tocount. At the end of this counting phase, i.e. at the trailing edge ofsaid pulse appearing at the output of said multivibrator 53, themonostable multivibrator 56 generates a pulse illustrated in FIG. 5,line 5C, causing the result of this counting step to be written-in inregister 40 and to become available at outputs 41-0 to 41-7. The valueof this number is the product of the frequency by the counting time,i.e. in the present instance: F₂ × D still equal to k × U × D. Since kand D are constants, the number is in fact proportional to U. It isconstantly available as an output, except during the short write-inpulse time in register 40.

Now, the actual mode of operation of the control device according to thepresent invention as illustrated in FIG. 2 can be explained withreference to the diagrams of FIG. 6 showing the signals available at theprinciple points of the device. The voltage U produces in fact twomagnitudes: on the one hand a signal F₁ through converter 9 which in thespecific case of the above-described application undergoes a linearvariation as a function of U but can also vary in other fashion withoutimpairing the character of the device, and on the other hand a binarynumber a through the medium of converter 10. This number varies fromzero (when U has its minimum value) to A (when U has its maximum value).

In the form of embodiment illustrated A = 100 but its value may in factdiffer from this figure. This signal having a frequency F₁ delivers viafrequency divider 17 and monostable multivibrator 18 a pulse each timeA-pulses having said frequency F₁ have been generated. This signal isillustrated in FIG. 6-A as registering with the signal F₁ of line 6-B.Each pulse of signal F₁ /A received from the monostable multivibrator 18energizes the bistable flip-flop 19 and causes the number a fromconverter 10 to be written in into the down counter 13. The latter willthus down-count a pulses of signal F₁ received at 14 to generate at theend of the counting period a pulse at its retaining output 16. Thispulse shown in FIG. 6-D is used for resetting the flip-flop 19 of whichthe output signal may be shown at 6-D. The same signal is used forcontrolling the power circuit 2 controlling in turn the energization ofmotor 1 of FIG. 1.

Proof must be given that the cycle ratio of this signal is definitelyproportional to U whatever the value of F₁ may be. In fact, thefrequency of the output signal is given by the activation signal offlip-flop 19. It is therefore equal to F₁ /A. Its period is thus T =A/F₁. The energization time of the flip-flop is linked to the timerequired for the counter 13 to down-count a/a pulses having a frequencyF₁. This time period is τ = a/F₁. The cyclic ratio R of the outputsignal is expressed as the ratio of its duration to its period, i.e.:##EQU2##

The value of F₁ is not a determining one in the expression of thisratio. Since a is proportional to the incoming voltage U, and A isconstant, the ratio R depends only on U, which is the intended purpose.

Of course, the form of embodiment described hereinabove with referenceto the accompanying drawings should not be construed as limiting thepresent invention, since, inter alia, for this specific form ofembodiment it is possible to use commercially available complexintegrated circuits affording both an easy construction and a highdegree of reliability.

What is claimed as new is:
 1. A device for controlling the supply ofcurrent to a direct-current motor by chopping said direct current bymeans of a control signal of the "hit or miss" type, wherein the cyclicratio of said control signal is a linear function of a referencemagnitude consisting of a control voltage U, comprising means forproviding a reference magnitude comprising a control voltage U, meansfor generating the frequency of the hit or miss control signal, meansfor determining the conduction time of said signal, and coupling meansfor coupling said means for generating the frequency of the hit or misscontrol signal to the means for determining the conduction time of saidsignal in such a manner that the cyclic ratio of said signal is a linearfunction of said reference magnitude, said means for generating thefrequency of the hit or miss control signal comprises avoltage-frequency converter receiving said control voltage U at itsinput, and said means for determining the conduction time of said signalcomprising an analog-digital converter receiving said control voltage Uat its input and generating a binary number at its outputs.
 2. A controldevice according to claim 1, wherein said voltage-frequency convertergenerates a frequency, said means for generating a frequency furthercomprises a frequency divider, means for connecting the output of saidvoltage-frequency converter to said frequency divider, a monostablemultivibrator, and means for connecting the output of said frequencydivider to said monostable multivibrator.
 3. A control device accordingto claim 2, wherein said means for determining the conduction time ofsaid signal further comprises a pulse down-counter having its inputsconnected to the corresponding outputs of said analog-digital converter,a clock input connected to the output of said voltage-frequencyconverter and a loading input connected to the output of said monostablemultivibrator, said coupling means comprises a bistable flip-flop havingone input connected to the output of said monostable multivibrator, theoutput signal of said bistable flip-flop constituting the hit or misscontrol signal to be fed to said direct-current chopper.
 4. A controldevice as claimed in claim 3 wherein said bistable flip-flop furthercomprises a second input connected to the output of said pulse downcounter.
 5. A control device as claimed in claim 4 wherein said meansfor providing a reference magnitude comprises means for providing acontrol voltage U proportional to the supply of current in a D.C. motor.